Dynamic Curing Agents for Amine-Hardened Epoxy Vitrimers with Short (Re)processing Times

Abstract

This work presents a straightforward strategy to introduce highly dynamic and adaptable cross-links into common epoxy resin formulations. For this, an oligomeric amine-based curing agent containing vinylogous urethane (VU) bonds was developed. This novel polyfunctional amine curing agent can be used as a drop-in solution for existing epoxy resin technologies, resulting in transparent, rigid and, at the same time, highly reprocessable catalyst-free epoxy vitrimers. The oligomeric VU curing agents are prepolymerised via a straightforward condensation reaction between acetoacetates extended with different classical amine monomers and epoxy hardeners. It is found that vitrimer properties can be readily introduced into these epoxy formulations by converting less than 50 mol% of the hardener's amine functionalities into dynamic vinylogous urethane bonds. In this way, epoxy vitrimers can be obtained with material properties comparable to ones of the VU-free epoxy formulations. In addition, remarkably short processing times are observed in the absence of any catalyst, and the material displayed very short stress relaxation times and good recyclability, actually representing the most performant VU-based vitrimers so far. Furthermore, a proof of concept for its use in obtaining glass fiber-reinforced epoxy composites is also presented.

Vitrimers: directing chemical reactivity to control material properties

“…58 Recently, the idea was further taken up by Nicolaÿ and co-workers in polybutadiene vitrimers 59 and by our group in vinylogous urethane epoxy-based vitrimers. 60 While this concept represents an effective 'generic' strategy to suppress creep, the reprocessability also suffers qualitatively and quantitatively, and it seems hard to overcome this intrinsic limitation. Nevertheless, targeted irreversible cross-linking is a straightforward way to improve some of the adverse properties of a vitrimer.…”

Investigation of the cure kinetics and thermal stability of an epoxy system containing cystamine as curing agent

“…The most common curing agents used for epoxy systems are Diethylenetriamine (DETA), Hexamethylenediamine (HMDA), Diaminodiphenylsulfone (DDS), Methylenedianiline (MDA), hexahydrophthalic anhydride (HHPA), Phthalic anhydride (PA), Triethylenetetramine (TETA) and dicyandiamide (DICY), which are classified according to the structure (aromatic or aliphatic), toxicity (toxic or non‐toxic), physical state (liquid or solid), curing temperature and other relevant properties 9 . In recent years, considerable attention has been paid to the design and to use of novel curing agents for epoxy resins with features such as flame retardancy, 10 self‐restoration ability, 11 vitrimeric properties 12 and fast curing 13 . Curing agents based on bio‐resources are also abundant in the literature 14,15 …”

Dually Crosslinked Polymer Networks Incorporating Dynamic Covalent Bonds

“…Another approach to design DDNs is to integrate dynamic bonds into well-known thermoset systems, such as epoxy resins. This was recently achieved by incorporating disulfide [ 43 , 155 ], imine [ 50 ], or vinylogous urethane moieties [ 156 ] into common epoxy or diene networks. By integrating DCBs, properties like recyclability and reprocessability can be imparted to typical thermosetting materials.…”